Fuel cells combine hydrogen and oxygen without combustion to form water and to produce direct current electric power. The process can be described as reverse electrolysis. Fuel cells have potential for stationary and portable power applications; however, the commercial viability of fuel cells for power generation in stationary and portable applications depends upon solving a number of manufacturing, cost, and durability problems.
Electrochemical fuel cells convert fuel and an oxidant to electricity and a reaction product. A typical fuel cell consists of a membrane and two electrodes, called a cathode and an anode. The membrane is sandwiched between the cathode and anode. Fuel, such as hydrogen, is supplied to the anode, where an electrocatalyst catalyzes the following reaction: 2H2→4H++4e−.
At the anode, hydrogen separates into hydrogen ions (protons) and electrons. The protons migrate from the anode through the membrane to the cathode. The electrons migrate from the anode through an external circuit in the form of an electric current. An oxidant, in the form of oxygen or oxygen-containing air, is supplied to the cathode, where it reacts with the hydrogen ions that have crossed the membrane and with the electrons from the external circuit to form liquid water as the reaction product. The reaction is typically catalyzed by the platinum metal family. The reaction at the cathode occurs as follows: O2+4H++4e−→2H2O.
The successful conversion of chemical energy into electrical energy in a primitive fuel cell was first demonstrated over 160 years ago. However, in spite of the attractive system efficiencies and environmental benefits associated with fuel-cell technology, it has proven difficult to develop the early scientific experiments into commercially viable industrial products. Problems have often been associated with lack of appropriate materials that would enable the cost and efficiency of electricity production to compete with existing power technology.
Proton exchange membrane fuel cells have improved significantly in the past few years both with respect to efficiency and with respect to practical fuel cell design. Some prototypes of fuel-cell replacements for portable batteries and for automobile batteries have been demonstrated. However, problems associated with the cost, activity, and stability of the electrocatalyst are major concerns in the development of the polymer electrolyte fuel cell. For example, platinum (Pt)-based catalysts are the most successful catalysts for fuel cell and other catalytic applications. Unfortunately, the high cost and scarcity of platinum has limited the use of this material in large-scale applications. The development of low temperature polymer electrolyte membrane fuel cells is furthermore severely hampered by the fact that the oxygen reduction reaction (ORR) is slow, resulting in low catalytic activities, even using platinum as a catalyst.
In addition, a problem with the use of platinum at the anode has been the poisoning of the catalyst surface by carbon monoxide impurities. On the cathode side, usually higher catalyst loadings have been utilised because methanol and other carbon containing fuel passing through the membrane react with oxygen at the cathode under catalytic effect of platinum, thereby decreasing the efficiency of the fuel cell.
To improve the catalytic efficiency and reduce the cost, other noble metals and non-noble metals are used to form Pt alloy as catalysts. Noble metals including Pd, Rh, Ir, Ru, Os, Au, etc. have been investigated. Non-noble metals including Sn, W, Cr, Mn, Fe, Co, Ni, Cu (U.S. Pat. No. 6,562,499) have also been tried. Different Pt-alloys were disclosed as catalysts for fuel cell applications. Binary alloys as catalysts include Pt—Cr (U.S. Pat. No. 4,316,944), Pt—V (U.S. Pat. No. 4,202,934), Pt—Ta (U.S. Pat. No. 5,183,713), Pt—Cu (U.S. Pat. No. 4,716,087), Pt—Ru (U.S. Pat. No. 6,007,934), Pt—Ti, Pt—Cr, Pt—Mn, Pt—Fe, Pt—Co, Pt—Ni, Pt—Cu (GB 2 242 203). Ternary alloys as catalysts include Pt—Ru—Os (U.S. Pat. 5,856,036), Pt—Ni—Co, Pt—Cr—C, Pt—Cr—Ce (U.S. Pat. No. 5,079,107), Pt—Co—Cr (U.S. Pat. No. 4,711,829),Pt—Fe—Co (U.S. Pat. No. 4,794,054), Pt—Ru—Ni (U.S. Pat. No. 6,517,965), Pt—Ga-(Cr, Co, Ni) (U.S. Pat. No. 4,880,711), Pt—Co—Cr (U.S. Pat. No. 4,447,506). Quaternary alloys as catalysts include Pt—Ni—Co—Mn, (U.S. Pat. No. 5,225,391), Pt—Fe—Co—Cu (U.S. Pat. No. 5,024,905).
However, for the PEM fuel cell to become a viable technology there is still a need to increase the catalytic activity or decrease the cost of the electrodes. Since the cost of the expensive ion conducting membrane separating the electrodes scales with the geometric area/active-site density of the electrode, the reduction of cost by using cheaper but less active electrodes with lower active-site density would be offset by the increasing cost of the membrane. Moreover, a decreased active site density cannot be offset by utilizing an electrode with a greater thickness: this would also impede the transport of reactive gases. As an example, reference should be made to the so-called Fe/C/N electrodes as disclosed inter alia by Lefevre et al., Science, 324, 71(2009). They have turnover frequencies, i.e. the number of electrons produced per active site per second, comparable to platinum electrodes, but still have lower active-site density.
Japanese patent application JP 10 214630 A discloses the use of binary alloys containing noble metals and rare earth metals in polymer electrolyte fuel cells. The alloys according to this application contain 20% or more by weight of “intermetallic compounds”. This is interpreted to mean that 20% or more of the alloy exists in a single ordered phase. It is not entirely clear what constitutes the remaining part of the alloy, but it would appear to be the constituent metals in a ratio that is not very well defined. The entire alloy acts as the electrocatalyst.
Korean patent application KR 2003 0030686 discloses a metal cathode for an electron tube comprising a metal alloy having, as a main component, Pt5La, Pt3Sc, Pt2Ti, Pt4Y, Pt3Y, Pt5Hf, PtEr, or Pt5Ce, and 0.1 to 20% by weight of one or more metals selected from the group consisting of molybdenum, tantalum and tungsten. The cathode further comprises 0.5 to 25% by weight of one or more elements selected from the group consisting of barium, strontium, and calcium. There is no indication, however, that the cathode may be useful for other uses than for an electron tube.
Accordingly, it is an object of the invention to provide an electrode alloy material with an increased catalytic activity towards oxygen reduction compared to pure platinum. It is furthermore an object of the invention to provide an electrode alloy with a lower cost compared to pure platinum while retaining a comparable active-site density. Another object of the invention is to provide an electrode alloy material whose activity enhancement over Pt is stable over extended periods of time.